The invention concerns axial-flow cooling arrangements in rotating electrical machines, and in particular, though not exclusively, axial-flow cooling arrangements in a large cage induction machine employing exclusively air cooling of the stator and rotor.
Many large rotating machines employ what is known as radial ventilation for cooling the stator and rotor. An example of this is shown in FIG. 1. In FIG. 1, a squirrel cage induction motor 10 comprises a stator core 11 and a rotor core 12, each having a number of sections 13 and 14, respectively. Both cores are made up of a large number of laminations. The stator core 11 is firmly attached to a housing 15 and the rotor core 12 is secured to a shaft 16 which may drive a load (not shown). The stator core 11 is provided with a 3-phase winding 17, while the rotor core 12 is equipped with solid aluminium or copper rotor bars 18. The rotor bars 18 are shorted together in end rings, one at each end of the rotor. The stator core 11 and rotor core 12 are provided with radial ducts 19 through which air is forced in order to cool the stator and rotor.
Air inside the machine 10 is made to circulate round the machine by a fan 20 secured to the shaft 16. The air is drawn in through a number of ducts 21 situated between the rotor core and the shaft 16, and at the same time through an airgap 22 between the stator and rotor, the air then passing through the ducts 19 and over the rear 23 of the stator core, before being returned to the fan 20. On the way from the fan 20 to the air inlet points of the stator and rotor, the air passes through a heat exchanger 24, cooling air being supplied from outside the machine by a further fan 25 also mounted to the shaft 16.
The laminations of both the stator core and rotor core appear as shown in a simplified representation in FIG. 2. The stator laminations comprise a body portion 31 and a number of teeth 32. (In practice, the number of teeth in a large rotating machine would be much greater than the number shown in the figure). Between the teeth 32 are slots 33 into which the 3-phase winding 17 (see FIG. 1) is inserted.
The laminations of the rotor are constructed in a manner similar to those of the stator, i.e. each lamination consists of a body portion 35 and a number of teeth 36 and slots 37. The slots 37 are suitable shaped to receive the solid bars 18.
While radial ventilation has been used to good effect in large machines, it presents a number of drawbacks. Firstly, the use of a radial construction makes it difficult to achieve low vibration levels. This is because of the necessity to have, on the one hand, a special rotor arm construction for securing the rotor to the shaft while at the same time creating the duct spaces 21 for cooling the rotor, and, on the other hand, duct spacers (which may be in the form of "I" beams) separating the individual rotor core sections to create the radial ducts 19. Both these elements may create out-of-balance forces in the rotor assembly during operation, leading to vibration problems. Secondly, since there are radial ducts 19 in both the stator and the rotor, a "siren" noise effect can be produced when the machine is running, especially if the two sets of ducts are in line with each other. This can be reduced by offsetting the ducts on the stator relative to those on the rotor, or by employing a different number of ducts on the stator and the rotor. However, this causes complexity of manufacture and adds to the cost of the machine. Thirdly, the necessity for ventilation ducts 21 between the rotor core 36 and shaft 16 means that the rotor diameter is increased, which in turn increases the windage loss of the machine. Fourthly, there is an increased risk of airgap sparking. Fifthly, the forces exerted on the above-mentioned duct spacers (e.g. "I" beams) in a high-speed machine can cause these members to be dislodged from the end laminations to which they are supposed to be secured.
Because of these drawbacks, a technique known as axial ventilation has also been used. One known axial ventilation arrangement is shown in FIG. 3. In this arrangement, a ventilation duct 40 is provided in each of the teeth of the stator 11 adjacent to the airgap 22. The duct 40 runs the entire axial length of the stator, and air is forced through this duct in order to cool the laminations of the stator core and the windings 17. Additional cooling may be provided by forcing air through small ducts 41, 42 made in the body of the stator core and rotor core, respectively.
This technique enjoys the advantages of axial cooling, which include reduced windage due to the fact that the rotor 12 can be of smaller diameter, but suffers from the disadvantages manifest by the use of the duct 40, as will now be explained with the aid of FIG. 4.
FIG. 4 is a partial view of the arrangement of FIG. 3 showing a stator tooth 32 and two associated stator slots 33. Each slot 33 comprises a winding section 51 and a ventilation duct section 52, which represents the duct 40 in FIG. 3. The ventilation duct section 52 is sometimes termed a "tunnel slot". The winding section 51 accommodates the stator winding 53, which in this example is composed of two sections 54, each made from a number of rectangular conductors held together by a suitable binding means. The two sections 54 are kept apart by a separator 55. The winding 53 is prevented from moving down the slot 33 by a wedge 56 which runs the length of the stator core 11, or core section 12.
The use of such "tunnel slots" in such an axial ventilation system makes for inefficient cooling. This is for several reasons: firstly, the cross-sectional area of the tunnel slot 52 is relatively small, which restricts the flow rate of the cooling air and produces an undesirably large pressure drop along the axis of the stator. The tunnel slot 52 may be increased in depth to allow a greater throughput of air, but with this must go a reduction in depth of the winding section 51 in order not to prejudice too much the mechanical properties of the stator core. This in turn means that the winding 53 must be made shorter and fatter, which necessitates a longer end-winding 17 in order to satisfy minimum clearance requirements at the end-winding itself. Secondly, the surface area of the tunnel slot 52 in contact with the air is restricted, which impairs the cooling efficiency of the arrangement. Thirdly, the top part 57 of the winding 53 and its adjacent lamination portions have long heat flow paths 58 to the tunnel slot 52, which produces an undesirably high temperature gradient between these two parts of the slot.
FIG. 4 also shows a pair of rotor bars 18 occupying the slots 37 of the rotor 12.
Axial airflow has also been employed in a very large synchronous machine as an adjunct to the water cooling of the stator winding. This arrangement is shown in FIG. 5. In FIG. 5, which shows a stator tooth 32 and two adjacent slots 33, the stator tooth 32 is provided with two small ducts 61, 62 running the whole length of the stator core. These ducts serve to provide nominal axial air cooling of the stator laminations only. Cooling of the stator winding (not shown) in the slots 33 is achieved by arranging for the conductors of the winding to be hollow and forcing water through them.
In a further known axial cooling arrangement, a cage rotor 12 (see FIG. 6) is provided with small-diameter air ducts 71 in the rotor teeth 37. In order to supplement the inadequate cooling effect that this measure produces, the arrangement incorporates in addition larger ducts 72 situated in the body of the rotor core below each rotor bar 18. Thus the ducts 71 perform essentially cooling of the laminations with some cooling of the rotor bars 18, while ducts 72 take away heat mainly from the rotor bars 18.
It is an object of the invention to provide a rotating electrical machine with axial cooling of the stator and/or rotor which seeks to overcome or mitigate the drawbacks associated with the above known axial cooling arrangements.